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Latin American Journal of Pharmacy
(formerly Acta Farmacéutica Bonaerense)
Trabajos originales
Recibido el 5 de marzo de 2007
Aceptado: 31 de marzo de 2007
Lat. Am. J. Pharm. 26 (4): 507-12 (2007)
Stabilization of Foams by Amphiphilic
Crystalline Solids and Liquid Crystals
Ricardo C. PASQUALI * & Carlos BREGNI
Departamento de Tecnología Farmacéutica, Facultad de Farmacia y Bioquímica,
Universidad de Buenos Aires, Junín 956, 6° piso (1113) Buenos Aires, Argentina.
SUMMARY. In this work, it was experimentally studied the influence over the stability of foams on
aerosol of concentrates with pearlescent aspect and, in some cases, with liquid crystalline characteristics,
too. The pearlescent, such as the formation of liquid crystalline structures, is the result of the interaction
of surfactants with fatty acids or long chain alcohols. Coincidentally with investigations of other authors, it
was found that pearlescent concentrates produce very stable foams. It was also determinate that the
pearlescent it is not a consequence of liquid crystalline phases, as other authors supposed, but because of
solid crystals.
RESUMEN. “Estabilización de Espumas en Aerosol por Sólidos Cristalinos Anfifílicos y Cristales Líquidos”. En
este trabajo se estudió experimentalmente la influencia sobre la estabilidad de espumas en aerosol de concentrados con aspecto nacarado y, en algunos casos, además con características líquida cristalinas. Tanto el nacarado
como la formación de estructuras líquidas cristalinas son resultantes de la interacción de tensioactivos con ácidos
grasos o alcoholes de cadena larga. Coincidentemente con investigaciones de otros autores se encontró que los
concentrados nacarados dan espumas muy estables. También se determinó que el nacarado no se debe a la presencia de fases líquida cristalinas, como suponían otros autores, sino de cristales sólidos.
INTRODUCTION
Miles et al. 1,2 observed that the speed of liquid drainage through foams of sodium lauryl
sulfate diminished significantly with the aggregate of lauryl alcohol. This diminution of the
drainage speed was attributed to the high viscosity resulting of the formation of a complex
between sodium lauryl sulfate and lauryl alcohol. The hypothesis of complex formation was
formulated initially by Schulman & Rideal 3,
who announced, in 1937, that when a
monomolecular film of cetyl alcohol or cholesterol was scattered over an aqueous solution of
sodium cetyl sulfate, the cetylsulfate anions penetrated the alcohol film and formed a complex.
Becher & Del Vecchio 4 obtained similar results
to the ones obtained by Miles, Shedlovsky and
Ross using ethoxylated lauryl alcohol joined
with lauryl alcohol or cetyl alcohol. Investigat-
ing with the system formed by sodium lauryl
sulfate, lauryl alcohol and water, Hume 5 observed the presence of abundant crystals that
persisted over the melting point of the alcohol
and the Krafft point of the soap. These crystals
melted between 29 °C and 32 °C to give an
isotropous solution which, at higher temperature, transformed into a turbid system with liquid crystalline characteristics.
Sanders 6 observed that, in aerosols formed
by surfactants and water, the stability of the
emulsions of the propellant on the concentrate
and of the foams discharged increased with the
presence of polar substances such as long chain
alcohol or fatty acids. Sander attributed these results to the formation of complex between the
molecules of the surfactants and the polar substances. On another paper, Sanders 7 got structures of pearlescent aspects in systems formed
KEY WORDS: Aerosols. Foams. Liquid crystals. Pearlescent structures.
PALABRAS CLAVE: Aerosoles. Cristales líquidos. Espumas. Nacarado.
*
Author to whom correspondence should be addressed. E-mail: [email protected]
ISSN 0326-2383
507
PASQUALI R.C. & BREGNI C.
by ethoxylated long chain alcohols and water.
In some cases, the pearlescent aspect keeps on
the presence of the propellant. Sanders attributed this optic effect to the formation of complex
that possibly had a liquid crystalline structure. In
1970, this author 8 assumed that as well as in
the surface of the propellant emulsion droplet
as in the foams on aerosol, the complex had a
liquid crystalline structure. Later, in the same paper, Sanders contradicts himself, when proposing that the complexes are found in the solid
state and that they stabilized emulsions and
foams in the same way that some small inorganic solids particles do. In the same paper, the author suggests again that the pearlescent is the
result of liquid crystalline structures. The hypothesis, which says that molecular complex
stabilize foams on aerosols and emulsions, was
objected by Friberg et al. 9 and by Jederström et
al. 10, who proposed that the stabilization was
because of the presence of a liquid crystalline
structures. In 1996, Goutev & Nickolov 11
demonstrated, through the Raman dispersion,
the presence of two phases on commercial
shave foam: a laminar gel phase, formed by
molecules of stearic acid (and stearic anions)
with all the hydrocarbon chains in trans position
and with a hexagonal order between them, and
a liquid isotropous phase. According to these
authors, the unilamellar structures, which are
initially on the concentrate, gradually organize
themselves in big multilamellar structures closed
around the bubbles.
The influence of the aggregate of small solid
particles over the formation and stability of
aqueous foams stabilized by surfactants depend
on the characteristics of the surfactant and on
the size and concentration of the particles 12.
While for surfactants the HLB value is the principal characteristic, in the case of amphoteric
particles that are adsorbed in the gas-water interfaces (foams) or water-in-oil (emulsions), the
relevant parameter is the contact angle θ between the particle and the interface. For hydrophilic particles, θ is generally lower than 90°
and an important part of the surface is in conIngredient
Cetyl alcohol
Propylparaben
Polyethylene glycol 400 dilaurate
Methylparaben
Water
S1
S2
4.8
0.03
7.64 % 7.64
0.10 % 0.07
92.26 % 87.42
%
%
%
%
%
Table 1. Composition of the concentrates S1 y S2.
508
tact with water. For hydrophobic particles, θ is
generally bigger than 90° and each particle is
more in contact with air or oil than with water.
In the first case, the solid particles stabilized
aqueous foams and emulsions of the oil in water type, while in the second one stabilized
aerosols and emulsions of water in oil type.
In this investigation, it was experimentally
studied the influence over the stability of
aerosol foams of concentrates with pearlescent
aspect and, in some cases, with liquid crystalline
characteristics, too.
MATERIALS AND METHODS
Cetyl alcohol is from Godrej Industries Ltd
(India). Stearic acid was provided by Materia
Oleochemicals (Argentina); its average relative
molecular mass, calculated through the acid value, is equal to 270. Polyethylene glycol 400 dilaurate is from Vasana (Argentina), polyoxyl 10
oleyl ether (Brij 97) is from Uniquema, polyoxyl
20 cetyl ether (Dehydol TA 20) is from Cognis
and sodium lauryl sulfate (minimum purity 95%)
was obtained from Mallinckrodt Chemical
Works. Triethanolamine, of commercial grade,
fulfils the established requirements on the Farmacopea Argentina VI. Methylparaben and
propylparaben were provided by Ueno Fine
Chemicals Industry Ltd (Japan).
Composition of the concentrates
The concentrates are dispersions of surfactants in water with or without the aggregate of
stearic acid or cetyl alcohol. These last substances are united to the surfactants and form
insoluble crystalline compounds. Concentrates
S1 and S5 was prepared in cold and with stirring until dissolution of surfactant and methylparaben. Concentrates S2, S3, S4, S6, S8, S10
and S11 were prepared adding, with stirring, the
oleous phase on aqueous phase and concentrates S7 and S9 were prepared mixing the components at 70-75 °C until dissolution.
The concentrate S1 is an aqueous dispersion
that contains 0.01 mol of polyethylene glycol
400 dilaurate in 100 grams. The S2 has also 0.02
mol de cetyl alcohol (Table 1).
The concentrate S3 has equimolecular quantities of stearic acid and triethanolamine (0.0185
mol of each one in 100 grams) and the S4 contains, in moles, the double of stearic acid according to triethanolamine (Table 2).
The concentrate S5 posses 0.02 mol of sodium lauryl sulfate per 100 grams, while the S6
has, also, 0.02 mol of cetyl alcohol (Table 3).
Latin American Journal of Pharmacy - 26 (4) - 2007
Ingredient
Stearic acid
Propylparaben
Triethanolamine
Methylparaben
Water
S3
5.00
0.03
2.75
0.07
92.15
S4
%
%
%
%
%
10.00
0.03
2.75
0.07
87.15
Ingredient
%
%
%
%
%
Table 2. Composition of the concentrates S3 y S4.
Ingredient
Cetyl alcohol
Propylparaben
Polyoxyl 10 oleyl ether
Methylparaben
Water
S7
S8
7.09 %
0.1 %
92.81 %
2.42
0.03
7.09
0.07
90.39
%
%
%
%
%
Stearic acid
Propylparaben
Polyethylene glycol 400 dilaurate
Triethanolamine
Methylparaben
Water
4.84
0.03
5.76
0.07
89.30
%
%
%
%
%
Cetyl alcohol
Propylparaben
Polyoxyl 20 cetyl ether
Methylparaben
Water
S9
S10
11.22 %
0.1 %
88.68 %
2.42
0.03
11.22
0.07
86.26
%
%
%
%
%
Table 5. Composition of the concentrates S9 y S10.
The filling of aerosols was doing in Summit de
Sudamérica SRL (Argentina).
S11
10.00
0.03
5.00
2.75
0.07
82.15
5.76 %
0.1 %
94.14 %
S6
Table 3. Composition of the concentrates S5 y S6.
Ingredient
Table 4. Composition of the concentrates S7 y S8.
Ingredient
Cetyl alcohol
Propylparaben
Sodium lauryl sulfate
Methylparaben
Water
S5
%
%
%
%
%
%
Table 6. Composition of the concentrate S11.
The concentrate S7 posses 0.01 mol of polyoxyl 10 oleyl ether per 100 grams. On the concentrate S8 was incorporated 0.01 mol of cetyl
alcohol, too (Table 4).
The concentrate S9 contains 0.01 mol of
polyoxyl 20 cetyl ether per 100 grams. On the
concentrate S10 it was added 0.01 mol of cetyl
alcohol (Table 5).
The concentrate S11 contains 0.0370 mol of
stearic acid, 0.0185 mol of triethanolamine and
0.0065 mol of polyethylene glycol 400 dilaurate
per 100 grams (Table 6).
Aerosols filling
The aerosol valves, provided by Summit de
Sudamérica SRL (Argentina), possessed a body
(code 97300) with an orifice of an internal diameter of 2.03 mm on the extreme of insertion of
the deep tube and a stem (code 77250) with
two lateral orifice of 0.46 mm and an exit orifice
of 0.66 mm. The actuator (code 77814) is the
one used for shaving foam. The propellant
used, denominated A-46, is a mixture of
propane and isobutane. The aerosols contain
100 g of concentrate and 5.6 g of propellant.
Essays done over the foams
Stability
It was discharged approximately 1 gram of
foam of each aerosol over two filter papers
Whatman 91 of 18.5 cm of diameter, collocated
one over the other. It was observed the persistence of the foam alter 1 min, 5 min, 15 min, 30
min, 1 h, 2 h and 3 h. This methodology was
proposed by Sanders 6.
Drainage
In order to evaluate drainage, a technique
proposed by Sanders 6 was used. It was discharged 5.0 grams of foam on a funnel of glass
of 5.1 cm of internal diameter, with an angle of
60 grades and a stem of 5 cm large and 4 mm of
internal diameter. It was determined the mass of
drained liquid after 1 min and then, alter intervals of 1 min until 10 min passed; and, finally,
at 15 and 30 min. On foams with high stability,
it was measured the drainage during a maximum lapses of 5 h with intervals of half an
hour. Graphically it was determined the time at
which the 50% of the mass of each foam
drained.
Microscopic observation
It was used an optic microscope Arcano
model XSZ-107 E provided of a graduated ocular and a photographic camera; and a polarizing
microscope Nikon model HFX-DX, with photographic camera, too. The concentrates were observed with ordinary light and with crossed polarizers.
509
PASQUALI R.C. & BREGNI C.
RESULTS
Concentrates characteristics
S1 is a fluid liquid, white and translucent. At
the polarizing microscope, it appears like an
isotropic emulsion. There are observed gouts of
1 to 10 µm of diameter. S2 is a liquid of greater
viscosity than S1, white, opaque and a bit
pearlescent. At the microscope, there are observed crystals surrounded by a similar structure
than the one observed in crystals with a spiral
growing With the crossed polarizers it is distinguished an abundant amount of extinction
crosses, typical of the crystal liquid laminar
phase. S3 is a fluid liquid, white and pearlescent. At the microscope, aciculate crystals can
be observed, not very contrasted with the back.
S4 is a fluid liquid, white and pearlescent. At the
microscope crystals with spiral growing with 50
µm of diameter can be observed, not very contrasted with the back. S5 is a transparent fluid
liquid. At the polarizing microscope it looks like
isotropic. S6 is a fluid liquid, white and pearlescent. At the polarizing microscope there are observed polygonal crystals producing double refraction with a spiral growing (Figs. 1 and 2). S7
is a fluid liquid, transparent and isotropic. In the
case of S8, at the first sight, two fluid liquid
phases are distinguished: the inferior one is
colourless and the superior one is white and
pearled. At the microscope; long aciculate crystals producing double refraction are observed,
that are disposed parallel between themselves.
There are also observed crystals with an approximately circular shape. S9 is a transparent
isotropic liquid. In S10. at the first sight, two fluid liquid phases are distinguished: the inferior
one is colourless and the superior one is white
and pearled. At the microscope aciculate crystals
producing double refraction are observed; they
present a polygonal shape with spiral growing.
In S11, at the first sight, two fluid liquid are distinguished: the inferior one is colourless and the
superior one is white and pearled. At the microscope long aciculate crystals are observed,
which disposed parallel between themselves,
similar to the ones of the concentrate S8. There
con also be seen polygonal crystals, not very
contrasted with the back, with and without spiral growing; with crossed polarizers, it presents
a non geometrical texture (Fig. 3).
Characteristics of the foams
Stability
The most stable foams are those which present a pearlescent aspect.
Figure 1. Microphotographies of the concentrates.
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Latin American Journal of Pharmacy - 26 (4) - 2007
Figure 4. Drainage of the foams S5, S7 y S9.
Figure 2. Microphotographies of the concentrate S6:
(a) with parallels polarizers; (b) with crossed polarizers.
Figure 5. Drainage of the foams S1, S2, S3, S8 y S10.
Drainage
On the foams produced by concentrates S4,
S6 and S11, it was not observed drainage during
the 5 h that lasted the essay. The drainage
curves of the remaining foams are shown in
Figs. 4 and 5. On Table 7, there are shown the
time in which drained 50% of the mass of each
foam.
Figure 3. Microphotographies of the concentrate S11:
(a) with parallels polarizers; (b) with crossed polarizers.
DISCUSSION
From the experimented concentrates, four of
them were isotropic (S1, S5, S7 and S9), five
presented solid crystals in suspension (S3, S4,
S6, S8 and S10) and in two of them were distinguished simultaneously solid particles and liquid
crystals (S2 and S11).
In all the concentrates with pearlescent aspect (S2, S3, S4, S6, S8, S10 and S11) were observed solid crystals. Consequently, it can be
concluded that the pearlescent aspect is given
because of the presence of solid crystalline particles.
All the isotropic concentrates (S1, S5, S7 and
S9), which are dispersions of surfactants of high
HLB values in water, produced foams with low
stability and high speed of drainage. These results are concordant with the ones obtained by
Sanders. The other concentrates possess solid
511
PASQUALI R.C. & BREGNI C.
Concentrate
Stability
of the foam (h)
Time in which
drained 50% of
the mass of foam (h)
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
0.25-0.5
>3
>3
>3
0.25-0.5
>3h
0.083-0.25
>3h
0.25-0.5
>3h
>3h
0.75
> 5.0
4.0
> 5.0
0.094
> 5.0
0.031
2.0
0.10
> 5.0
> 5.0
Table 7. Stability and drainage of the foams.
crystalline particles in suspension and produce
foams of high stability and low speed of
drainage. These concentrates, with exception of
S3, are formed by dispersions of surfactants and
cetyl alcohol or stearic acid. The concentrate S3
is constituted by an aqueous dispersion of a surfactant (triethanolamine stearate) resulting of the
reaction of equimolecular quantities of stearic
acid and triethanolamine However, and in spite
of not having an excess of stearic acid according
to triethanolamine, it presents a pearlescent aspect. The origin of this optic phenomenon
could be given to the formation of a solid complex produced between triethanolamine stearate
and stearic acid by hydrolysis of part of the anion stearate.
C17H35COO¯ + H2O → C17H35COOH + OH¯
512
Acknowledgment. The authors thank for the financing of this study provided by UBACyT, Project BO 43,
and to Martín Santero, of Summit of South America
SRL (Argentina), by the filling of the aerosols and the
provision of valves and cans, and to Guillermo Cozzi,
of the Mining Geologic Service of Argentina (SEGEMAR), to allow us to use the used polarizing microscope in this work.
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